1. Field of the Invention
This invention relates to a method of and apparatus for compensating signal drift during inspecting of the crown ring of containers for detecting chips and other defects which render containers unfit for use.
2. Description of the Prior Art
The existing apparatus for detecting chips and defects in container crown rings that uses infrared beams is subject to errors which reduce performance below the theoretical capability thereof. The errors arise because fixed threshold is used in the detector circuit. The sensitivity of the detector must be reduced from the maximum so that there will not be an unacceptable high false rejection rate due to component drift. This drift may arise because LED output efficiency changes with temperature and aging, or transistor gain changes with temperature, or transistor output changes due to ambient light variations, or detector threshold changes with temperature, or LED current changes due to changes in the drive resistors caused by high power, aging, humidity and temperature, or buildup of dirt and moisture on the protective windows which reduces the light reaching the phototransistor. Moreover, if a single threshold value is used, all LED phototransistors channels must track as they drift or imbalances will arise. There are many variables involved which makes any drift cancellation technique extremely complicated.
The pertinent prior art includes the prior U.S. Pat. No. 4,213,042 of Beach and Smith which issued July 15, 1980 and is commonly assigned with this application. This patent discloses emitters and receivers in cooperating pairs to scan an entire crown ring, electronic control means driving the emitters, sequential driver circuits and synchronous detector circuits, and means to trigger a reject device.
The published British patent application No. 2,003,268A, dated Mar. 7, 1979, discloses a method of inspecting transparent objects by projecting infrared radiation onto the object, detecting the emerging beam, and applying synchronous control circuits for scanning the sources and detectors.
The commonly assigned patent of Gender et al., U.S. Pat No. 3,631,255 of Dec. 28, 1971 has disclosed a crown ring inspection apparatus which includes photoelectronic means for examining transparent containers in association with an optical projector for presenting the container image to a photomultiplier tube which develops electrical signals suitable for operating a reject mechanism.
The prior art includes Calhoun et al. U.S. Pat. No. 3,349,906 of Oct. 31, 1967; Kidwell U.S. Pat. No. 3,479,514 of Nov. 18, 1969; Mathias U.S. Pat. Nos. 3,267,796 of Aug. 23, 1966, and 3,355,980 of Dec. 5, 1967; and Sorbie U.S. Pat. No. 3,327,849 of June 27, 1967.